Periodic or random vibrations or shock can excite the resonant frequencies in a circuit article such as a circuit board, flexible circuit, etc. which can be problematic due to the resultant formation of undesirable stresses, displacements, fatigue, mechanical forces, and even sound radiation. Such undesirable vibrations or shocks are typically induced by external forces and can be experienced by a wide variety of circuit articles and under a variety of conditions. For example, printed circuit boards (PCBs) have the electrical traces laid-out upon or within a base substrate or laminate and typically have various components such as integrated circuit (I.C.) chips, resistors, capacitors, and the like, placed on and connected to these various electrical traces. Resonant vibrations can cause problems in circuit articles such as printed circuit boards (PCBs) and cards, etc., by significantly increasing the mechanical displacement of the base substrate or laminate, which may result in undesirable stresses and fatigue and subsequent premature circuit article failure.
Various techniques have been used to reduce vibrational and shock amplitude effects on circuit articles. Circuit articles such as circuit boards are typically protected from such shock or vibrations by a variety of methods such as (1) the addition of dampers to the exterior surface of the circuit article, i.e. "add-on dampers"; (2) by isolation of the circuit article or the structure in which it is used; and (3) by use of circuit article stiffeners. The use of these vibration and shock control methods can add significant cost and/or complexity to the structure in which the circuit article is incorporated.
Certain of these techniques, e.g. add-on dampers, utilize viscoelastic materials in exterior surface damping treatments for vibration and shock control. Two types of exterior surface damping treatments are commonly used: (a) free layer exterior damping treatment; and (b) constrained layer exterior damping treatment. Both of these exterior damping treatments can provide high levels of damping to a structure, i.e., dissipation of undesirable vibrations, without sacrificing the stiffness of the structure. Examples of such damping techniques are described, for example, in U.S. Pat. Nos. 2,819,032 (issued Jan. 7, 1953); 3,071,217 (issued Jan. 1, 1963); 3,078,969 (issued Feb. 26, 1963); 3,159,249 (issued Dec. 1, 1964); and 3,160,549 (issued Dec. 8, 1964).
As mentioned above, one method for controlling vibration is to isolate the circuit article from the vibration source. U.S. Pat. No. 4,053,943 (issued Oct. 11, 1977) describes a system for externally isolating a printed circuit board. Laminated damping elements are remotely positioned from and connected to the circuit board by post structural means fastened at each end to the circuit board and to the laminated damping element. Such an external isolation system adds to the overall size of the circuit board and may be impractical where close positioning of the circuit board to other structures is desired. Another isolation method is described WO 92/21178. By increasing the natural frequency of the circuit board, isolation can be achieved, again requiring complex design.
Free layer damping treatment is also referred to as "unconstrained layer" or "extensional damping" treatment. In this technique, damping occurs by applying a layer of viscoelastic damping material to an external surface of a structure. The material can be applied to one or both sides of a structure. The mechanism by which this treatment method dissipates undesirable energy, e.g., resonant vibrations, involves deformation. That is, when the structure is subjected to cyclic loading, for example, the damping material is subjected to tension-compression deformation and dissipates the energy through an extensional strain mechanism.
Constrained layer damping treatment is also referred to as "shear damping" treatment. For a given weight, this type of damping treatment is generally more efficient than the free layer damping treatment. In this technique, damping occurs by applying a damper consisting of one or more layers of viscoelastic damping material and one or more layers of a higher tensile modulus material to the external surface of a structure. That is, this damping technique is similar to the free layer damping treatment wherein a viscoelastic material is applied to an exposed surface of a structure, the difference being that the viscoelastic material is additionally constrained by a layer having a higher modulus than the viscoelastic material, e.g., a metal layer, in the constrained layer treatment. Energy dissipates from the viscoelastic damping material via a shear mechanism that results from constraints by the higher modulus constraining layer and the base structure.
Although these exterior surface damping techniques are used, the degree of damping is often times limited by thickness or spacing requirements, obstructions on the circuit article surface which limits application coverage, or accessibility or environmental limitations. For example, if the circuit article is desired to be a component in a size limited application, such as hard disk drives for portable computer systems, computers, calculators, pagers, cellular phones, etc. the ability to adequately damp the circuit articles by means of an "add-on" surface treatment damper or by isolating the circuit article within the structure may not be possible due to overall thickness requirements to meet space requirements or that the circuit article surface is nearly filled with components and a damper could not be added to the circuit article surface. Thus, an alternative approach is needed to damp vibrational or shock energy without adversely affecting the overall size or thickness of the circuit article or the structure incorporating the circuit article and by a process which is also cost effective.